function counter1(){
    return {number:0}
}
function counter2(){
    return {number:0};
}
let reducers = {
    counter1,
    counter2
}
let rootReducer;
//这就是我们合并后的根状态，或者说总状态对象
let rootState =  {
    counter1:{number:0},
    counter2:{number:0}
}

function counter1({number}){
    return <div>{number}</div>
}
//把总状态映射成子对象，并让此子对象成为组件的属性对象
const mapStateToProps = state=>state.counter1;//{number:0}
//你把mapStateToProps传给了connect函数
//在connect内部先获取到总状态rootState。把此总状态传给mapStateToProps
//得到返回值，返回值总会成为组件的属性对象